1. Field of the Invention
The present invention relates to an exhaust gas purifying device for an engine.
2. Description of the Related Art
It is well known that an engine in which a lean air-fuel mixture is burned is provided with a catalyst having an oxidizing ability arranged in the exhaust passage of the engine. In the engine, the unburned hydrocarbon (HC) and carbon monoxide CO contained in the exhaust gas are oxidized in the catalyst in the oxidizing atmosphere and converted to H.sub.2 O and CO.sub.2. However, the exhaust gas flowing into the catalyst also contains sulfur dioxide SO.sub.2 and the SO.sub.2 is converted, in the catalyst in the oxidizing atmosphere, to sulfur trioxide SO.sub.3. If the SO.sub.3 reacts with H.sub.2 O in the catalyst, H.sub.2 SO.sub.4 is synthesized.
It is undesirable that the H.sub.2 SO.sub.4 is discharged from the catalyst in the form of the mist of sulfuric acid. To solve this problem, Japanese Unexamined Patent Publication No. 53-100314 discloses an exhaust gas purifying device for an engine in which the reducing agent is fed to the catalyst to reduce SO.sub.3 and H.sub.2 SO.sub.4, i.e., sulfates, in the catalyst to SO.sub.2.
To reduce the sulfates in the catalyst sufficiently, it is required that the concentration of the reducing agent in the exhaust gas flowing into the catalyst is always made relatively high. However, in a case where the lean air-fuel mixture is burned in the engine, the large amount of the reducing agent is needed to make the concentration of the reducing agent in the inflowing exhaust gas high and, if such a feeding of the reducing agent is performed continuously, a large amount of the reducing agent is needed.